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Ref.: Proc. OCEANS MTS/IEEE ' 2006,
(ISBN:), p., Boston,USA, September 2006. (to appear)
Abstract : This paper presents experimental results for a
passive time reversal experiment
conducted off the west coast of Portugal during the MREA'2004 mission. A single acoustic
projector generated a 2/4-PSK stream at 200 and 400 baud, modulated around a
carrier frequency of 3600 Hz. The signals were received at a range of
about 5 Km on a vertical array with 8 hydrophones placed at depths of
10, 15, 55, 60, 65, 70, 75, 80 m.
Time reversal - both active and passive - has aroused
considerable interest recently in communications applications as a mean
of partially compensating for the intersymbol interference (ISI)
introduced by the channel at the receiver [1]. The results of several
experiments at sea reported in the technical literature suggest that
time reversal by itself will not ensure reliable detection of the
transmitted symbols, and must be complemented by adaptive equalization
at the receiver (see also the theoretical results of [2]). Nonetheless,
this computationally inexpensive technique is useful because, when
compared with full multichannel equalization, it allows simpler
equalizers to be used [3]. Arguably, the resulting overall reduction in
complexity at the receiver more than makes up for the moderate
degradation in performance.
In a few papers that have been published on simultaneous
equalization and time reversal the two systems are operated in tandem,
i.e., time reversal creates a single-channel signal which is then
independently processed by an equalizer. This configuration is
inevitable in active time reversal, where the ocean itself generates
the refocused waveform. In passive time reversal, however, the signals
received at an array of hydrophones are synthetically combined, and may
therefore be individually postprocessed after convolving them with
estimates of the time-reversed channel impulse responses.
The analysis of MREA'04 data has shown that, when the
synthetically recombined signal exhibits poor signal-to-interference
(ISI+noise) ratio, this can often be attributed to the individual
multichannel components interfering destructively in spite of a
seemingly appropriate temporal alignment. We propose to multiply each
of them by an adaptive complex term before summing, so that
constructive interference can be restored. The parameters of these
rotators are jointly optimized with the downstream adaptive equalizer
coefficients at each symbol interval to minimize the output MSE. In the
limit when the downstream equalizer has no feedforward filter, the
structure can be thought of as a low-complexity multichannel combiner
that takes advantage of a useful physical property of sound propagation
in the ocean to drastically reduce the number of adaptive parameters to
be estimated.
In the MREA'04 setup the 8 unevenly-spaced array sensors are
separated by a minimum of 5 m, or about 12 wavelengths at the carrier
frequency. Under these conditions, which significantly differ from the
classic half-wavelength separation in array processing, the question
arises as to whether focusing occurs by coherent beamforming, or mainly
as a result of spatial diversity across the array. While that question
is not answered thoroughly in the paper, a statistical characterization
of this SIMO channel is performed that provides clues on the spatial
correlation. Moreover, results are presented for different subarray
sizes when studying the performance of the various demodulation
structures to assess the impact of the spatial dimension.
In addition to the proposed multichannel receiver structure, our
results illustrate the performance of plain time-reversal,
time-reversal followed by single-channel adaptive equalization, and
multichannel equalization with and without reduced-complexity
multichannel combining.
References:
[1] G. Edelmann et al., "An initial demonstration of underwater
acoustic communication using time reversal", IEEE JOE 27(3), pp.
602-609, Jul. 2002.
[2] M. Stojanovic, "Retrofocusing techniques for high-rate
acoustic communications",
JASA 117(3), Pt. 1, pp. 1173-1185, Mar. 2005.
[3] A. Silva, S. Jesus, J. Gomes, and V. Barroso, "Underwater
acoustic communications using a 'virtual' electronic time-reversal
mirror approach", in P. Chevret and M. Zakharia, editors, 5th European
Conference on Underwater Acoustics, pp. 531-536, Lyon, France, June
2000.
ACKNOWLEDGMENT: this work was partially supported by
FCT projects NUACE - POSI/CPS/47824/2002 and RADAR -
POCTI/CTA/47719/2002.
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